The bacterium Deinococcus radiodurans is able to resist doses of ionizing radiation many times higher than those that kill other organisms. In the January 10 Science, Smadar Levin-Zaidman and colleagues at the Weizmann Institute of Science, Israel, report that the D. radiodurans genome exists in an unusual ring-like structure that may account for its radioresistance, by restricting the diffusion of radiation-generated free DNA ends (Science, 299:254-256, January 10, 2003).

Scanning electron microscopy analysis revealed that D. radiodurans cells have a tetrad morphology with each quarter containing equal amounts of DNA (each contains a single copy of the bacterial genome). This compartmentalization suggests that DNA repair after radiation does not involve homologous recombination. The bacterial nucleoids adopted a toroidal morphology that presumably dictates a rigid structure facilitating template-independent, error-free, end-joining of DNA breaks.

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